The hidden matzah.
At the Passover seder table, three matzahs are placed in a stack.
Near the beginning of the seder, the middle matzah is broken in two pieces, and the larger piece of this matzah is called the afikomen. It is saved to be eaten after the meal.
Many families have the custom of hiding the afikomen--either the parents hide it and the children search for it, or the children hide it and parents search. A prize is often given to whoever locates this important piece of matzah. The seder cannot continue until the afikomen has been located and consumed.